Kaltuun Warsame Charlotte Lockwood Alexander Badrick Miriam Tarragona Turu Ying Xiang Chua Melanie Poonai Frah Abdullahi*
To understand the mechanism of polymorphism land snails were studied, Cepaea nemoralis, which are also known as grove snails, in order to recognize the different forms of variation within a species. Cepaea nemoralis is well known for the rich polymorphism of its shell and has been studied for decades. These snails are frequently used in biology to understand genetics as their shells are simply inherited, and also polymorphic. Polymorphism in biology occurs when two or more clearly distinguishable phenotypes exist in the same population of a species. Cepaea nemoralis is highly polymorphic in shell colour, apart from the peristome (lip) of the adult snail which remains a dark brown colour throughout. The colour series includes brown, pink and yellow. The differences in the frequency of the colour vary in different habitats and areas, reflecting selection by thrush anvils predators and other birds that locate their prey visually.Phenotypic variability also means that there are different banding systems of up to five bands. Banding may be absent or found in reduced forms of one, two, three or four bands. The Cepaea nemoralis is also popular due to its widespread nature; it occupies a wide range of habitats including the woodlands, hedgerows and grassland. The study was conducted at Pulpit Hill nature reserve, near Monk’s Riseborough, Buckinghamshire. The national trial describes the area as: “a gem of a nature reserve with areas of unimproved chalk grassland covered in wildflowers and woodland”. The site of study was spread across and down the hill, allowing testing at “high” and “low” altitudes and in grassland and woodland. The hill was relatively steep and the area of study extended roughly 250m down and 150m across. On the day of the study the weather was misty and chilly, with some light showers. Jones et al (1977) investigated genetic polymorphism, more specifically the shell pattern, in the land snail Cepaea. This species of land snail is a hermaphrodite and cross-fertilizes with other Cepaea species. The link of the C. nemoralis shell polymorphisms to genetics was studied and shell characteristics such as shell colour and presence of bands have been attributed to a supergene.The distribution of morph frequency was also considered and three general patterns were found – fluctuations of morph frequency in low marshland areas, segregation between populations in areas of slightly higher areas and unvarying frequencies in high areas. Predation was also found to have an effect on selection of polymorphisms – woodland areas had a high frequency of brown and pink shells due to the background of brown leaves that exists in these areas. The brown and pink shells of snails allow predation avoidance in these areas as they camouflage with the background. The design of the method investigated the independent variables: altitude and environment. These two aspects of habitat were studied to determine their effect on the phenotypic evolution of the snail species, Cepaea nemoralis. To observe the effect of altitude, the area was divided into high and low regions, to independently contrast the population number of each phenotypic variation: pink, yellow and brown-shelled snails and the number of bands on their shell, determining if altitude caused phenotypic change within the species. The high altitude region was further sub-categorised by environment: grassland and woodland. A second variable was studied to determine if altitude alone caused phenotypic variation or if it was an accumulation of different variables. Within each region for each variable, three replicates were taken to illustrate that the results were not a cause of genetic drift, but selection. The experiment aimed to show selection acting on the Cepacea species, showing that a phenotypic variant was selected more favourably when compared to others and thus had a specific greater presence in the population. If the sampling results show a bias to a particular phenotype, it may indicate the act of selection on this particular phenotype and thus a greater reproduction rate and quantity in following generations, as it is seen as being more advantageous and successful for offspring. Genetic drift should also be considered to be a cause of any sampling results as it may be due to chance sampling and genetic drift, which suggest that any findings are due to random events. This effect would be reflected in findings which show no significant difference between the phenotypic variants of the species and the variables considered. To eliminate the possibility and influence of any sampling errors, the sampling method was replicated three times within matching habitats to ensure a range of data could be collected to generate accurate and valid results. Cepaea is a very opportunistic snail which rapidly invades suitable habitats. [It] often lives in disturbed and transient habitats in which there must be frequent population extinctions and recolonizations.” (Jones et al. 1977)This means that every Cepaea population will vary widely in phenotypic frequency and in almost no populations would it be possible to explain the genetic variations observed by either only selection or only genetic drift but rather “complex and perhaps unique explanations are needed for almost every Cepaea population.” (Jones et al. 1977). Discussion The results of the study illustrated a greater frequency of pink shells at higher altitudes than brown and yellow shells. This is due to the lightness of the color, pink shell snails populated regions with high light intensity - high altitude - because their pink color causes heat from light to be less readily absorbed into the shell, and so the need to remain in light exposed areas. In contrast, brown and yellow shell snails can populate regions with lower light intensity in low altitudes as their darker colored shells absorb heat from light more readily, meaning they do not need to be constantly exposed in light. An additional hypothesis was also suggested, which was that in high altitude more snails were found in the grassland habitat, whereas in low altitude, more were found in the woodland. The pink shelled Cepea nemoralis were at a greater frequency in the woodland, whereas the brown and yellow shelled snails were found to recur more in the grassland, when combining both altitudes. This may be due to the fact that pink snails require shelter from predators such as the thrush anvil, and do so by concealing themselves in woodland. Whereas yellow and brown Cepea nemoralis are darker coloured and thus not as easily detected, meaning that they can live in grassland.
This study’s findings are concordant with the previous literature (Jones et al. 1977), which found morphological differences between Cepea nemoralis found at different altitudes and in different habitats. Snail shell color was significantly different in slightly higher areas: dominantly pink shells at the higher altitude, and yellow shells at the lower altitude. Jones et al. (1977) found that in woodland areas, a greater frequency of brown and pink shells were found – offering an evolutionary advantage by blending in with their habitat, which these results also confirm to an extent, as more pink shells were found in woodland. Although these results also deny this as more brown shells were found in grassland. On completion of the investigation, some limitations were observed. The experiment was conducted in a relatively dry environment with minimal rainfall. However, optimal conditions for snails include rain, because they migrate to the surface of the soil in order to avoid flooding. Therefore the weather on the day of experimentation hindered the amount of Cepaea nemoralis that could be studied. The experiment also required the color phenotype identification of the snails, however in some instances it was hard to distinguish between the types, which can lead to a false recording of the phenotype. Based on these limitations, suggested improvements are to couple the timing of the investigation with preferred weather reports to ensure increased presence of Cepaea nemoralis.
To conclude, results showed a significant preference of habitats between the forms of the Cepea nemoralis species. It was suggested that their phenotype indicates and determines the altitude and form of land that the snails predominantly populate and affects their success to progress to the next generation as a result of selection. References Jones, J.S. Leith, B.H. and Rawlings, P. (1977) Polymorphism in Cepaea: a problem with too many solutions? Annual Review of Ecology, Evolution and Systematics.8 109-143
Members:
Kaltuun Warsame
Charlotte Lockwood
Alexander Badrick
Miriam Tarragona Turu
Ying Xiang Chua
Melanie Poonai
Frah Abdullahi*
To understand the mechanism of polymorphism land snails were studied, Cepaea nemoralis, which are also known as grove snails, in order to recognize the different forms of variation within a species. Cepaea nemoralis is well known for the rich polymorphism of its shell and has been studied for decades. These snails are frequently used in biology to understand genetics as their shells are simply inherited, and also polymorphic. Polymorphism in biology occurs when two or more clearly distinguishable phenotypes exist in the same population of a species. Cepaea nemoralis is highly polymorphic in shell colour, apart from the peristome (lip) of the adult snail which remains a dark brown colour throughout. The colour series includes brown, pink and yellow. The differences in the frequency of the colour vary in different habitats and areas, reflecting selection by thrush anvils predators and other birds that locate their prey visually. Phenotypic variability also means that there are different banding systems of up to five bands. Banding may be absent or found in reduced forms of one, two, three or four bands. The Cepaea nemoralis is also popular due to its widespread nature; it occupies a wide range of habitats including the woodlands, hedgerows and grassland.
The study was conducted at Pulpit Hill nature reserve, near Monk’s Riseborough, Buckinghamshire. The national trial describes the area as: “a gem of a nature reserve with areas of unimproved chalk grassland covered in wildflowers and woodland”. The site of study was spread across and down the hill, allowing testing at “high” and “low” altitudes and in grassland and woodland. The hill was relatively steep and the area of study extended roughly 250m down and 150m across. On the day of the study the weather was misty and chilly, with some light showers.
Jones et al (1977) investigated genetic polymorphism, more specifically the shell pattern, in the land snail Cepaea. This species of land snail is a hermaphrodite and cross-fertilizes with other Cepaea species. The link of the C. nemoralis shell polymorphisms to genetics was studied and shell characteristics such as shell colour and presence of bands have been attributed to a supergene. The distribution of morph frequency was also considered and three general patterns were found – fluctuations of morph frequency in low marshland areas, segregation between populations in areas of slightly higher areas and unvarying frequencies in high areas. Predation was also found to have an effect on selection of polymorphisms – woodland areas had a high frequency of brown and pink shells due to the background of brown leaves that exists in these areas. The brown and pink shells of snails allow predation avoidance in these areas as they camouflage with the background.
The design of the method investigated the independent variables: altitude and environment. These two aspects of habitat were studied to determine their effect on the phenotypic evolution of the snail species, Cepaea nemoralis. To observe the effect of altitude, the area was divided into high and low regions, to independently contrast the population number of each phenotypic variation: pink, yellow and brown-shelled snails and the number of bands on their shell, determining if altitude caused phenotypic change within the species. The high altitude region was further sub-categorised by environment: grassland and woodland. A second variable was studied to determine if altitude alone caused phenotypic variation or if it was an accumulation of different variables. Within each region for each variable, three replicates were taken to illustrate that the results were not a cause of genetic drift, but selection.
The experiment aimed to show selection acting on the Cepacea species, showing that a phenotypic variant was selected more favourably when compared to others and thus had a specific greater presence in the population. If the sampling results show a bias to a particular phenotype, it may indicate the act of selection on this particular phenotype and thus a greater reproduction rate and quantity in following generations, as it is seen as being more advantageous and successful for offspring. Genetic drift should also be considered to be a cause of any sampling results as it may be due to chance sampling and genetic drift, which suggest that any findings are due to random events. This effect would be reflected in findings which show no significant difference between the phenotypic variants of the species and the variables considered. To eliminate the possibility and influence of any sampling errors, the sampling method was replicated three times within matching habitats to ensure a range of data could be collected to generate accurate and valid results.
Cepaea is a very opportunistic snail which rapidly invades suitable habitats. [It] often lives in disturbed and transient habitats in which there must be frequent population extinctions and recolonizations.” (Jones et al. 1977)This means that every Cepaea population will vary widely in phenotypic frequency and in almost no populations would it be possible to explain the genetic variations observed by either only selection or only genetic drift but rather “complex and perhaps unique explanations are needed for almost every Cepaea population.” (Jones et al. 1977).
Discussion
The results of the study illustrated a greater frequency of pink shells at higher altitudes than brown and yellow shells. This is due to the lightness of the color, pink shell snails populated regions with high light intensity - high altitude - because their pink color causes heat from light to be less readily absorbed into the shell, and so the need to remain in light exposed areas. In contrast, brown and yellow shell snails can populate regions with lower light intensity in low altitudes as their darker colored shells absorb heat from light more readily, meaning they do not need to be constantly exposed in light.
An additional hypothesis was also suggested, which was that in high altitude more snails were found in the grassland habitat, whereas in low altitude, more were found in the woodland. The pink shelled Cepea nemoralis were at a greater frequency in the woodland, whereas the brown and yellow shelled snails were found to recur more in the grassland, when combining both altitudes. This may be due to the fact that pink snails require shelter from predators such as the thrush anvil, and do so by concealing themselves in woodland. Whereas yellow and brown Cepea nemoralis are darker coloured and thus not as easily detected, meaning that they can live in grassland.
This study’s findings are concordant with the previous literature (Jones et al. 1977), which found morphological differences between Cepea nemoralis found at different altitudes and in different habitats. Snail shell color was significantly different in slightly higher areas: dominantly pink shells at the higher altitude, and yellow shells at the lower altitude. Jones et al. (1977) found that in woodland areas, a greater frequency of brown and pink shells were found – offering an evolutionary advantage by blending in with their habitat, which these results also confirm to an extent, as more pink shells were found in woodland. Although these results also deny this as more brown shells were found in grassland.
On completion of the investigation, some limitations were observed. The experiment was conducted in a relatively dry environment with minimal rainfall. However, optimal conditions for snails include rain, because they migrate to the surface of the soil in order to avoid flooding. Therefore the weather on the day of experimentation hindered the amount of Cepaea nemoralis that could be studied. The experiment also required the color phenotype identification of the snails, however in some instances it was hard to distinguish between the types, which can lead to a false recording of the phenotype. Based on these limitations, suggested improvements are to couple the timing of the investigation with preferred weather reports to ensure increased presence of Cepaea nemoralis.
To conclude, results showed a significant preference of habitats between the forms of the Cepea nemoralis species. It was suggested that their phenotype indicates and determines the altitude and form of land that the snails predominantly populate and affects their success to progress to the next generation as a result of selection.
References
Jones, J.S. Leith, B.H. and Rawlings, P. (1977) Polymorphism in Cepaea: a problem with too many solutions? Annual Review of Ecology, Evolution and Systematics. 8 109-143